1. Field of the Invention
This invention is related to designing gated electrode structures that aim to reduce overvoltages in electrochemical and photoelectrochemical cells by altering the field in the electrolyte near an electrode.
2. Description of the Related Art
Hydrogen is expected to become an important energy storage medium for renewable energy sources which have a fickle output due to variability of environmental factors and for mobile applications such as transportation. Cost effective techniques for obtaining hydrogen from water is a prerequisite for the hydrogen economy and several approaches are being explored. Electrolysis appears to be among the more attractive schemes for splitting water. Electrolysis could be applied directly at photoelectrodes, or electrical energy could be produced by solar cell arrays, wind turbines, or other means, and hydrogen produced by electrolysis in an electrochemical cell.
The schematic of a conventional electrochemical cell (100) for the electrolysis of water is shown in FIGS. 1(a)-(b). Ionic currents flow through the cell (100) and electron transfer occurs at the electrodes (101,102), such that hydrogen (103) is produced at the cathode (101) and oxygen (104) is produced at the anode (102) with appropriate choices of the electrolyte (105) and cell voltage (106). In the conventional cell (100), an electrode (101,102) is made of one material that might be a composite. FIG. 1(a) shows a face view of the electrode (101) and FIG. 1(b) shows an edge view of the electrodes (101) and (102).
For a working electrode which is an anode, electrons transfer from the electrolyte to the anode. For a working electrode which is a cathode, electrons transfer from the cathode to the electrolyte.
The standard potential for splitting water at 25° C. under 1 atmosphere (atm) pressure is 1.23 V. However, due to the various over-voltages required, the voltage across a cell with platinum electrodes and saturated sodium hydroxide (NaOH), for example, as electrolyte is about 2.0 V at room temperature for an electrolysis current density of 100 mA cm−2 (this voltage can be reduced to ˜1.6 V if the cell is operated at 125° C.). In fact, at room temperature, the operating cell voltage common for electrolysis of water is over 2 V, which is much higher than the thermodynamic standard potential of 1.23 V for splitting water. Therefore, lowering electrode overvoltage is very desirable for improving overall efficiency of these systems.
In the photoelectrode cell configuration, either one or both electrodes could be made of semiconducting material so that the photovoltaic effect can be utilized for generating the electromotive force (emf) used in driving electrolysis. With a single photoelectrode in a photoelectrochemical cell, it is necessary that the total cell potential needed for electrolysis be created in that electrode. This means that the photoelectrode should be made with a semiconducting material that has a bandgap greater than about 2.5 eV, if the voltage for electrolysis is about 2.0 V. This drastically reduces the number of photons available for the generation of photocurrent, and a tandem cell electrode design, or two cells in series, is required. In the case of pure electrochemical cells for electrolysis, overvoltages reduce conversion efficiencies. Hence, the need to design electrode structures that reduce cell overvoltages.
Several approaches have been found, in the open literature and patents, for reducing cell overvoltages at the electrodes. These have included (a) forming nanostructured coatings, such as covering the electrode surface with material such as platinum black, nanostructured alloys, nanostructured oxide semiconductors, carbon nanotubules, carbon nanotube (CNT) composite with other materials, nanotubular and porous semiconductors, and nanocomposites with fullerenes and conducting polymers, (b) using low work function metals, (c) using Metal-Insulator-Metal (MIM) tunnel structures to facilitate electron transfer to the radical that captures the electron to produce hydrogen, and (d) partially coating the surface with a hydrophobic layer to facilitate hydrogen evolution. Besides working on electrodes and electrolytes most appropriate for electrolysis, researchers have also investigated pulsed sources for electrolysis. However, to date, stainless steel for the cathode, nickel or platinized nickel for the anode, with saturated KOH or NaOH as the electrolyte, has been the standard cell configuration for industrial cells for the electrolysis of water.
In metal cathodes, electron transfer occurs from energies near the Fermi level of the metal to the highest energy states available in the electrolyte around the Fermi energy, when single step tunneling is possible, or by multi-step tunneling. Metal cathodes are not efficient photocathodes. Semiconductors can serve as cathodes as well as photocathodes.
FIGS. 2(a) and 2(b) are band diagrams. FIG. 2(a) shows a band diagram (200) of a semiconductor cathode made of a degenerate n-type semiconductor, wherein electrons (201) transfer (202) to surface states (203) and then to the hydronium ion(s) (204) in the electrolyte (205) (the present invention has just shown the hydronium ion as a possible radical to which electron transfer occurs for electrolysis, however, the level (206) to which electrons tunnel is still a subject of debate). FIG. 2(b) shows a band diagram (207) of conventional photocathodes made with p-type semiconductors.
FIGS. 2(a) and 2(b) show how semiconductor cathodes, in general, use the higher energy of electrons (201) in the conduction band (208) of the semiconductor to facilitate transfer (202) of electrons (201) to the radical (204), and the radical (204) decomposes to give hydrogen (209).
FIG. 2(a) shows how n-type semiconductors require to be heavily doped, in order to facilitate tunnel injection (202) of electrons (201) to surface states (203) and then to the radical (204) (for example, to the hydronium level(s) (206) involved with electrolysis).
The band diagram (207) of semiconductor photocathodes is shown in FIG. 2(b). FIG. 2(b) shows how photocathodes most widely discussed in literature use p-type material and utilize surface charges (210), at the semiconductor electrolyte interface (211), for the band bending (212) that brings about charge separation (213) of the electrons (201) and holes (214). The arrow (215) represents photoexcitation in the photocathode that with subsequent charge separation creates the cell current and voltage between the cathode and anode.
For both structures, shown in FIG. 2(a) and FIG. 2(b), the band bending (212) is determined by the semiconductor|electrolyte junction (211). The degenerately doped n-type semiconductor cathode does not seem to offer any special advantage over the metal cathode, although the degenerately doped n-type semiconductor cathode does provide a higher energy level for electrons compared to stainless steel. There is little discussion about this in the literature. N-type semiconductors with lower doping have been used as photo-anodes but have problems with oxidation. P-type semiconductors have been widely investigated for photocathodes.
Because the band bending is controlled by the semiconductor|electrolyte junction (211), there are some drawbacks in the overall concept in both schemes:                (a) the built-in voltage and field that bring about carrier separation are due to surface effects that are less reproducible, and the built-in voltage is lower than can be obtained with semiconductor homo-junctions or hetero-junctions made on a substrate with a certain bandgap,        (b) the energy level for electron transfer to the hydronium ion is not optimized and results in an overvoltage, and        (c) the electric field in the electrolyte next to the semiconductor|electrolyte interface is controlled by the surface charge and depletion charge in the semiconductor and is often controlled by the former.        
Use of nano-structured coatings helps increase the surface area and possibly local fields near sharp tips or nano-structured couples/half-cells. However, the nano-structured coatings do not significantly improve performance, i.e. reduce overvoltage at high current densities, over that of conventional metal electrodes. This is because none of the schemes listed above directly address the problem of altering the field and potential distribution in the electrolyte next to an electrode, and this is a parameter that needs investigation.
The present invention addresses the problem by designing electrodes that are sandwiches of several layers of materials, to form various classes of gated electrode structures that aim to reduce overvoltages by altering the charge, field and potential distribution in the electrolyte near the electrode.